Risk Assessment of Infectious Endogenous Banana Streak Viruses in Guadeloupe.

Infectious alleles of endogenous banana streak viruses (eBSVs) are present in the genome of all banana interspecific cultivars, including plantains and cooking types. Activation of these infectious eBSV alleles by biotic and abiotic stresses leads to spontaneous infections by cognate viruses and raises concerns about their ability to promote outbreaks of banana streak viruses under field cultivation conditions. We undertook a comprehensive risk assessment study of infectious eBSV alleles of species BSOLV, BSGFV and BSIMV in banana interspecific cultivars in Guadeloupe, a tropical island of the Caribbean where bananas are grown for export and local markets. We carried out a prevalence survey of BSOLV, BSGFV and BSIMV species in a range of cultivars grown in Guadeloupe. Our results suggest that BSOLV and BSGFV infections arise from the activation of infectious eBSVs rather than vector-borne transmission and point to a correlation between altitude and infection rates in interspecific hybrids with AAB genotypes. We studied the dynamics of activation of infectious eBSOLV and eBSGFV alleles by tissue culture and field cultivation in a range of cultivars. We showed that tissue culture and field cultivation trigger distinct activation pathways, resulting in distinct activation patterns. We also showed that activation decreased over time during cell culture and field cultivation and that BSV infections arising from the activation of infectious eBSV alleles cause symptomless infections in the most cultivated plantain in Guadeloupe, French Clair. Overall, our study shows that the risk of BSV outbreaks resulting from the activation of infectious eBSVs in plantain originating from vegetative multiplication is negligible in Guadeloupe.

Post-transcriptional regulation of several biological processes involved in latex production in Hevea brasiliensis.

BACKGROUND: Small RNAs modulate plant gene expression at both the transcriptional and post-transcriptional level, mostly through the induction of either targeted DNA methylation or transcript cleavage, respectively. Small RNA networks are involved in specific plant developmental processes, in signaling pathways triggered by various abiotic stresses and in interactions between the plant and viral and non-viral pathogens. They are also involved in silencing maintenance of transposable elements and endogenous viral elements. Alteration in small RNA production in response to various environmental stresses can affect all the above-mentioned processes. In rubber trees, changes observed in small RNA populations in response to trees affected by tapping panel dryness, in comparison to healthy ones, suggest a shift from a transcriptional to a post-transcriptional regulatory pathway. This is the first attempt to characterise small RNAs involved in post-transcriptional silencing and their target transcripts in Hevea. METHODS: Genes producing microRNAs (MIR genes) and loci producing trans-activated small interfering RNA (ta-siRNA) were identified in the clone PB 260 re-sequenced genome. Degradome libraries were constructed with a pool of total RNA from six different Hevea tissues in stressed and non-stressed plants. The analysis of cleaved RNA data, associated with genomics and transcriptomics data, led to the identification of transcripts that are affected by 20-22 nt small RNA-mediated post-transcriptional regulation. A detailed analysis was carried out on gene families related to latex production and in response to growth regulators. RESULTS: Compared to other tissues, latex cells had a higher proportion of transcript cleavage activity mediated by miRNAs and ta-siRNAs. Post-transcriptional regulation was also observed at each step of the natural rubber biosynthesis pathway. Among the genes involved in the miRNA biogenesis pathway, our analyses showed that all of them are expressed in latex. Using phylogenetic analyses, we show that both the Argonaute and Dicer-like gene families recently underwent expansion. Overall, our study underlines the fact that important biological pathways, including hormonal signalling and rubber biosynthesis, are subject to post-transcriptional silencing in laticifers.

Histone acetylation and the circadian clock: a role for the MYB transcription factor RVE8/LCL5.

Most organisms have developed an internal timing mechanism or circadian clock that is able to generate 24-hour biological rhythms in synchronization with the diurnal environmental changes. Despite our increasing understanding of the molecular machinery underlying circadian clock function, a complete picture of the components and regulatory mechanisms governing the circadian system in Arabidopsis thaliana is still lacking. In a recent study, we have characterized the role of the MYB-like transcription factor REVEILLE8/LHY-CCA1-LIKE5 (RVE8/LCL5) within the Arabidopsis circadian clock. We have generated RVE8/LCL5 mutant and overexpressing plants and showed that similar to the MYB-like transcription factor CIRCADIAN CLOCK-ASSOCIATED1 (CCA1), RVE8/LCL5 binds to the promoter of key clock component TOC1 (Timing of CAB expression 1) and regulates its circadian expression. However, the mechanisms of RVE8/LCL5 and CCA1 circadian function seem to differ: while CCA1 represses TOC1 expression by facilitating a hypo-acetylated state of Histone H3, RVE8/LCL5 contributes to TOC1 expression by favouring H3 acetylation at the TOC1 locus. Although CCA1 has a more predominant role on this regulation, our results showing the opposing function of RVE8/LCL5 open interesting questions about the complex networks of transcriptional regulators and chromatin remodeling activities that need to be integrated in synergistic and antagonistic ways to generate the circadian periodicity.

Functional implication of the MYB transcription factor RVE8/LCL5 in the circadian control of histone acetylation.

Despite our increasing understanding of the molecular determinants essential for circadian clock function, we still lack a complete picture of the mechanisms contributing to clock progression in plants. Here, we explore the role of REVEILLE8/LHY-CCA1-LIKE5 (RVE8/LCL5) within the Arabidopsis circadian system. RVE8/LCL5 encodes a MYB-like transcription factor similar to CIRCADIAN CLOCK-ASSOCIATED1 (CCA1) and ELONGATED HYPOCOTYL (LHY), which are essential regulators of the Arabidopsis circadian clock. Consistent with the sequence similarity, the rhythmic expression of RVE8/LCL5 shows a morning acrophase comparable to that of CCA1 and LHY. Plants mis-expressing RVE8/LCL5 display a variety of circadian phenotypes, including altered circadian gene expression and photoperiodic flowering time. Similar to CCA1, RVE8/LCL5 regulates the expression of the oscillator gene TOC1 (TIMING OF CAB EXPRESSION1) by associating with the TOC1 promoter and by modulating the pattern of histone 3 (H3) acetylation. However, the mechanisms of RVE8/LCL5 and CCA1 activity in this regulation differ markedly. Indeed, the use of chromatin immunoprecipitation and pharmacological inhibition assays reveals that RVE8/LCL5 favours a hyper-acetylated state of H3 at the TOC1 promoter, which may facilitate the rising phase of TOC1. In contrast, CCA1 represses TOC1 expression by promoting histone deacetylation. Thus, despite the sequence homology and the similar morning phase of expression, RVE8/LCL5 and CCA1 have opposing regulatory functions within the Arabidopsis circadian clock, although CCA1 has a more predominant role. We propose that contrasting chromatin compaction and transcriptional modulation through the opposing activities of RVE8/LCL5 and CCA1 might provide a fine-tuning mechanism for precisely shaping the TOC1 circadian waveform in Arabidopsis.

Natural synchronisation for the study of cell division in the green unicellular alga Ostreococcus tauri.

Ostreococcus tauri (Prasinophyceae) is a marine unicellular green alga which diverged early in the green lineage. The interest of O. tauri as a potential model to study plant cell division is based on its key phylogenetic position, its simple binary division, a very simple cellular organisation and now the availability of the full genome sequence. In addition O. tauri has a minimal yet complete set of cell cycle control genes. Here we show that division can be naturally synchronised by light/dark cycles and that organelles divide before the nucleus. This natural synchronisation, although being only partial, enables the study of the expression of CDKs throughout the cell cycle. The expression patterns of OtCDKA and OtCDKB were determined both at the mRNA and protein levels. The single OtCDKA gene is constantly expressed throughout the cell cycle, whereas OtCDKB is highly regulated and expressed only in S/G2/M phases. More surprisingly, OtCDKA is not phosphorylated at the tyrosine residue, in contrast to OtCDKB which is strongly phosphorylated during cell division. OtCDKA kinase activity appears before the S phase, indicating a possible role of this protein in the G1/S transition. OtCDKB kinase activity occurs later than OtCDKA, and its tyrosine phosphorylation is correlated to G2/M, suggesting a possible control of the mitotic activity. To our knowledge this is the first organism in the green lineage which showed CDKB tyrosine phosphorylation during cell cycle progression.

Starch division and partitioning. A mechanism for granule propagation and maintenance in the picophytoplanktonic green alga Ostreococcus tauri.

Whereas Glc is stored in small-sized hydrosoluble glycogen particles in archaea, eubacteria, fungi, and animal cells, photosynthetic eukaryotes have resorted to building starch, which is composed of several distinct polysaccharide fractions packed into a highly organized semicrystalline granule. In plants, both the initiation of polysaccharide synthesis and the nucleation mechanism leading to formation of new starch granules are currently not understood. Ostreococcus tauri, a unicellular green alga of the Prasinophyceae family, defines the tiniest eukaryote with one of the smallest genomes. We show that it accumulates a single starch granule at the chloroplast center by using the same pathway as higher plants. At the time of plastid division, we observe elongation of the starch and division into two daughter structures that are partitioned in each newly formed chloroplast. These observations suggest that in this system the information required to initiate crystalline polysaccharide growth of a new granule is contained within the preexisting polysaccharide structure and the design of the plastid division machinery.